The present invention relates to a method for rapidly characterising a tuneable laser.
The method can be applied to evaluate and select lasers with respect to emitted wavelengths, and to systematically find good operation points.
Tuneable semiconductor lasers include several sections to which current is injected, these sections typically being three or four in number. The wavelength, power and mode purity of the lasers can be controlled by regulating the current injected into the various sections. Mode purity implies that the laser is at an operation point, i.e. at a distance from a combination of the drive currents where so-called mode jumps take place and where lasering is stable and sidemode suppression is high.
In the case of telecommunications applications, it is necessary that the laser is able to retain its wavelength to a very high degree of accuracy and over long periods of time, after having set the drive currents and the temperature. A typical accuracy in this respect is 0.1 nanometer and a typical time period is 20 years.
In order to be able to control the laser, it is necessary to map the behaviour of the laser as a function of the various drive currents. This is necessary prior to using the laser after its manufacture.
One problem is that lasers exhibit an hysteresis. This means that for a given drive current set-up, i.e. a given operation point, the laser will deliver different output signals with respect to power and wavelength depending on the path through which the laser has passed with respect to the change in said drive currents in order to arrive at the operation point in question. Thus, a given drive current set-up will not unequivocally give the expected wavelengths or power in this case.
In the case of a tuneable laser, the wavelength of the light emitted is determined mainly by the current through or the voltage across the tuning sections. The power output is controlled by current to the gain section of the laser or by the voltage across said section.
All possible control combinations afforded by the tuning sections, or a subset of said sections, are investigated when characterising a laser. During the characterising process, the light emitted is studied with respect to wavelength and sidemode suppression and in controlling the gain section with regard to power adjustment.
The large number of possible control combinations, typically tens of billions, of which fewer than a hundred shall be selected, makes total mapping of the laser impossible in view of the large amount of data generated.
One method of characterising a laser is described in Swedish Patent No. 9900536-5. According to this patent publication, the hysteresis of the laser is also considered. The patent relates to a method of quickly sorting out control combinations that do not result in correct wavelengths.
The aforesaid patent relates to a method of evaluating a tuneable laser and of determining suitable operation points for a laser that includes two or more tuneable sections in which injected current can be varied and of which at least one is a reflector section and one is a phase section. According to the patent, part of the light emitted by the laser is led to an arrangement that includes a Fabry-Perot filter and a first and a second detector, said detectors being adapted to measure the power of the light and to deliver a corresponding detector signal. The detectors are arranged relative to the Fabry-Perot filter such that the detector signals will contain information relating at least to the wavelength of the detected light from a number of wavelengths given by the filter. The currents through the tuning sections are swept so as to pass through different current combinations, the ratio between the two detector signals being measured during said sweeps. The current combination is stored when the ratio between the detector signals lies within a predetermined range that indicates that the light emitted lies within one of a number of wavelengths given by the Fabry-Perot filter.
The Fabry-Perot filter is adapted to have a certain given transmission for those wavelengths included in a so-called channel plan where each channel corresponds to a well-defined wavelength.
One problem with the invention according to the aforesaid prior patent publication is that no information is obtained as to which operation point belongs to which channel. It is therefore necessary to determine and sort the various operation points, which must be done manually and with conventional instruments.
This problem is solved by the present invention.
The present invention thus relates to a method of evaluating a tuneable laser and determining suitable laser operation points, wherein the laser includes one or more tuneable sections in which injected current can be varied, and wherein the invention is characterised by leading part of the light emitted by the laser to an arrangement that includes a first filter in the form of a periodic filter that gives rise to a signal which varies periodically with wavelength, such as a Fabry-Perot filter, and a first and a second detector which are adapted to measure the power of the light and to emit a corresponding detector signal; in that the detectors are arranged relative to the periodic filter such that said detector signals will include information relating to at least the wavelength of the detected light from a plurality of wavelengths given by the filter; in that the second filter is disposed parallel with the periodic filter and comprises a filter whose response varies monotonically with the wavelength, wherein the light transmitted through the second filter is led to a third detector which emits a detector signal corresponding to the power of the light and therewith wavelength; in that the currents through the tuning sections are swept so as to pass through different current combinations; in that the ratio between the two detector signals is measured during each sweep; in that when the ratio between the detector signals lies within a predetermined range indicating that the light emitted lies within one of a number of the wavelengths given by the periodic filter, the third detector is caused to detect so as to sort out the wavelength concerned; and in that the control combination of said tuning currents is stored together with the wavelength concerned.